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| United States Patent |
5,738,704
|
|
Dorofeev
, et al.
|
April 14, 1998
|
Charging stock for steel production
Abstract
An improved charging stock for steel production which facilitates earlier
and more uniform carbon oxidation, and increases the rate of oxygen
transport in the melt, comprises and iron-carbon alloy having silicon
therein and an oxide-containing material. The iron-carbon alloy preferably
has a ratio of carbon to silicon in the range of approximately 4-40:1. The
ratio of oxide-containing material surface area to the weight of the
iron-carbon alloy is preferably maintained in the range of 5-100 m.sup.2
/ton.
| Inventors:
|
Dorofeev; Genrikh Alekseevich (Tula, RU);
Afonin; Serafim Zakharovich (Moscow, RU);
Sitnov; Anatolii Georgievich (Tula, RU)
|
| Assignee:
|
Intermet-Service and Company (Tula, RU)
|
| Appl. No.:
|
588111 |
| Filed:
|
January 18, 1996 |
Foreign Application Priority Data
| Current U.S. Class: |
75/304; 75/316; 75/572 |
| Intern'l Class: |
C21B 003/02 |
| Field of Search: |
75/316,572,304
|
References Cited
U.S. Patent Documents
| 2710796 | Jun., 1955 | Pinkerton | 75/25.
|
| 3948612 | Apr., 1976 | Schulten-Baumer | 75/572.
|
| 4564388 | Jan., 1986 | Vallomy | 75/12.
|
| 4581068 | Apr., 1986 | Schramm | 75/130.
|
| 4797154 | Jan., 1989 | Benedetti et al. | 75/46.
|
| 4957546 | Sep., 1990 | Lazcano-Navarro | 75/529.
|
| 5364441 | Nov., 1994 | Worner | 75/10.
|
| 5425797 | Jun., 1995 | Dorofeev et al. | 75/306.
|
| 5562753 | Oct., 1996 | de Lassat de Pressigny et al. | 75/10.
|
Primary Examiner: Andrews; Melvyn
Attorney, Agent or Firm: Heller Ehram White & McAuliffe
Claims
What is claimed is:
1. A composite charging stock for metallurgical processing, comprising:
an iron-carbon alloy, said alloy including silicon; and
an oxide containing material,
the ratio of the external surface area of said oxide-containing material to
the mass of said iron-carbon alloy being in the range of 5-100 m.sup.2 per
ton.
2. The composite charging stock of claim 1 where the ratio of carbon to
silicon in said iron-carbon alloy is in the range of 4-40:1.
3. The composite charging stock of claim 1 wherein said iron-carbon alloy
comprises pig iron.
4. The composite charging stock of claim 1 wherein said oxide-containing
material comprises iron ore pellets, iron ore, metal concentrates, scale,
agglomerate, pulverized wastes, sludge from metallurgical processes or
mixtures thereof.
5. A composite charging stock for metallurgical processing, comprising:
an iron-carbon alloy, said alloy including silicon, the ratio of carbon to
silicon in said iron-carbon alloy being in the range of 4-40:1; and
an oxide containing material,
the ratio of the external surface area of said oxide-containing material to
the mass of said iron-carbon alloy being in the range of 5-100 m.sup.2 per
ton.
6. The composite charging stock of claim 5 wherein said iron-carbon alloy
comprises pig iron.
7. The composite charging stock of claim 5 wherein said oxide-containing
material comprises iron ore pellets, iron ore, metal concentrates, scale,
agglomerate, pulverized wastes, sludge from metallurgical processes or
mixtures thereof.
Description
FIELD OF THE INVENTION
The present invention relates in general to ferrous metallurgy, and more
particularly to an improved charging stock used in the production of
steel.
BACKGROUND OF THE INVENTION
Casting machines for casting pigs with a filler (such as an oxide material)
are well know in the art. Such machines typically function as follows:
Pellets are charged into the casting machine in automated containers
approximately 1 m.sup.3 (in volume), raised by telphers and subsequently
loaded into bins. A ladle, containing molten iron, is fed to the casting
machine and tilted by a manipulator. Molten iron is then poured from a
ladle into the casting machine, which is ultimately fed into ingot molds.
Casting machine feeders are then lowered into working position (commonly,
to a stop in the ingot molds), the gates opened, and the pellets
discharged through the supply system into the ingot molds (charging
boxes). The conveyer drive is then turned on. As the conveyers move, the
feeders level out the pellets in the ingot molds. The ingot molds, filled
with pellets, move into position and are filled with iron.
A major drawback of the charge material produced by the above method is the
low rate of carbon oxidation. The low carbon oxidation rate typically
results from the presence of a relatively low rate of oxygen supply due to
the under-development of the oxide material-base metal (e.g., iron-carbon
alloy) interface. This deficiency is especially noticeable at the start of
melting, when the bath has a reduced temperature. The under-development of
the interphase surface and the low temperatures inhibit the oxidation of
carbon.
Another drawback of the charge material is the pronounced nonuniformity of
the carbon oxidation rate and, in many instances, the complete suspension
of this reaction due to the preferential oxidation of silicon. Under
conditions where there is insufficient oxidizing agent, the silicon draws
the available oxygen for its own oxidation, inhibiting or entirely
terminating the oxidation of carbon. The absence of a controlled ratio of
carbon to silicon in the conventional charge material thus leads to a wide
spread in the rates of decarborization and carbon content upon melting.
Therefore, what is needed is an improved charging stock for metallurgical
processing that (i) facilitates earlier and more uniform oxidation of
carbon and (ii) attains a high rate of decarborization at low bath
temperatures.
SUMMARY OF THE INVENTION
The present invention substantially reduces or eliminates the disadvantages
and shortcomings associated with prior art charge materials. The invention
provides an improved charging stock for metallurgical processing that
facilitates earlier and more uniform oxidation of carbon during the
melting process. The charging stock also increases the rate of oxygen
transport in the melt.
Accordingly, it is an object of the present invention to provide a charging
stock for metallurgical processing which facilitates earlier and more
uniform oxidation of carbon.
It is another object of the present invention to provide a charging stock
for metallurgical processing which attains a high rate of decarborization
at low bath temperatures.
It is another object of the present invention to provide a charging stock
for metallurgical processing which enhances the stability of the carbon
content during the melting process.
It is yet another object of the present invention to provide a charging
stock for metallurgical processing which increases the rate of oxygen
transport during the melting process.
In accordance with the above objects and those that will be mentioned and
will become apparent below, the improved charging stock in accordance with
this invention comprises an iron-carbon alloy having silicon therein and
an oxide-containing material. The iron-carbon alloy preferably has a ratio
of carbon to silicon in the range of 4-40:1. The ratio of the
oxide-containing material surface area to the weight of the iron-carbon
alloy is preferably maintained in the range of 5-100 m.sup.2 /ton.
The advantages of this invention are (i) earlier oxidation of the carbon by
the oxygen of the oxide material, (ii) high rates of decarborization at
the low bath temperatures (.about.1250.degree.-1300.degree. C.), (iii)
more uniform oxidation of carbon during the melting process, and (iv)
enhanced stability of the carbon content during the melting process.
BRIEF DESCRIPTION OF THE DRAWINGS
Further features and advantages of the improved charging stock for
metallurgical processing disclosed herein will become apparent from the
following and more particular description of the preferred embodiment of
the invention as illustrated in the accompanying drawings in which:
FIGS. 1-3 are graphical illustrations of the change in carbon content as a
function of the carbon oxidation rate for various ratios of the external
surface area of the oxide-containing material to the weight of iron-carbon
alloy according to the invention;
FIG. 4 is a graphical illustration of the decarborization rate as a
function of time for the charging stock according to the invention and a
conventional charge material.
DESCRIPTION OF PREFERRED EMBODIMENTS
The improved charging stock of the present invention comprises an
iron-carbon alloy and an oxide-containing material. The charging stock is
produced by conventional methodology, preferably, by pouring molten
iron-carbon alloy over "lump" oxide-containing material. According to the
invention, various oxide-containing materials may be employed within the
scope of this invention. For example, the oxide-containing materials my
comprise iron ore pellets, iron ore, metal concentrates, scale,
agglomerate, pulverized waste, sludge from metallurgical processes and
mixtures thereof.
A key characteristic of the improved charging stock of the invention is the
use of oxide-containing materials which have a larger surface area than
that commonly employed in prior art materials. According to the invention,
the ratio of the external surface area of the oxide-containing material to
the weight of the iron-carbon alloy is preferably in the range of 5-100
m.sup.2 /ton. As will be recognized by one having ordinary skill in the
art, knowing the initial granulometric composition of the oxide-containing
material and its relative amount in the charging stock, one can easily
obtain the ratio set forth above.
Another key characteristic of the charging stock of the invention is that
the ratio of silicon to carbon in the iron-carbon alloy is preferably
maintained in the range of 4-40:1. The specified ratio is preferably
maintained by varying the silicon content in the iron-carbon alloy, since
it is technically more difficult to control the carbon concentration in
high carbon melts.
Given the same composition of the charging stock and a constant ratio of
its principal components--the oxide-containing material and iron-carbon
alloy--increasing the surface area of the oxide-containing material will
substantially increase the specific surface per unit mass of the
iron-carbon alloy.
Further, the amount of oxygen that is supplied to the base metal (i.e., the
iron-carbon alloy of the charging stock) generally increases in proportion
to the increase in surface area of the oxide-containing material. Thus,
the presence of carbon in the base metal enables it to melt at relatively
low temperatures (.about.1170.degree.-1200.degree. C.),which is generally
below the bath temperatures during the initial period of converter and
electric melting. This ensures a high rate of carbon transport in the base
metal, even at the start of melting when the bath is still cold, thereby
eliminating carbon mass transport as a factor limiting the oxidation of
carbon. As a result, optimum conditions (discussed below) for accelerated
oxidation of the carbon are created.
The first condition created by the charging stock of the invention is the
formation of a highly developed phase contact surface between the solid
oxide-containing material and the molten iron-carbon alloy. According to
the invention, a maximum value (.about.90-100 m.sup.2 /ton) of the ratio
of the external surface area of the oxide-containing material to the
weight of the iron-carbon alloy is also required. This sharply intensifies
oxygen transport to the carbon reaction front and eliminates (from the
rate of carbon oxidation) the constraints imposed by the stage of oxygen
supply on the resulting (total) carbon oxidation rate.
The second condition for accelerated oxidation of carbon that is created by
the charging stock of the invention is the reduced melting point of the
iron-carbon alloy base, attained through the presence of carbon. According
to the invention, the iron-carbon alloy melts at an initial bath
temperature in the range of 1200.degree.-1300.degree. C. This ensures the
required rate of carbon supply to the interface of the oxide-containing
material at temperatures substantially below the melting point of the
charging stock, the melting point of iron, and the final temperature of
the metal at the outlet.
The noted conditions ensure the early commencement of carbon oxidation at
the reduced temperatures of the metal bath even while the source of oxygen
supply--the oxide-containing material--it is in the solid state, i.e., at
temperatures below the melting point of the solid oxidizing agent.
Oxidation of carbon in the low-temperature region also occurs at elevated
rates, characteristic of the case where liquid iron is blown with gaseous
oxygen. Physically, this means that the decrease in the rate of oxygen
supply from solid oxide-containing material due to the low rate of oxygen
diffusion in the solid is compensated by the large surface of the solid
and by shortening the path of oxygen diffusion to the site of the carbon
reaction.
In contrast to conventional charge materials where the rate of oxygen
sharply limits the carbon oxidation, the stage that limits the carbon
oxidation in the charging stock of the invention is the rate of heat
supply to the charging stock and the melting rate. The sharp increase in
the phase interface between the oxide-containing material and the
iron-carbon alloy melt, under conditions where the alloy has already fused
and is adequately heated, eliminates the stage of oxygen supply as a
limiting factor. A new factor, the thermal factor, specifically, the
melting rate of the charging stock, is now the limiting factor. This means
that diffusion kinetics, which govern the rate of transport of the oxygen
flow, gives way as a limiting factor to the rate of heat exchange between
the charging stock and the environment, i.e., to heat transfer.
According to the invention, the presence of a developed specific surface
area (the ratio of the external surface of the oxide-containing material
to the weight of the iron-carbon alloy) in the range 5-100 m.sup.2 /ton
facilitates the commencement of carbon oxidation with relatively high
rates, even in the early stages of the heat when the bath is still
relatively cold. Moreover, this oxidation takes place in the presence of
the silicon contained in the iron-carbon alloy.
If the specific surface area at the interface between the oxide-containing
material and the iron-carbon alloy is less than 5 m.sup.2 /ton, the rate
of increase of the oxygen supply is relatively low, producing an oxygen
deficiency. In this case, oxidation of silicon, which has a much stronger
affinity for oxygen than carbon, predominantly develops. As a result, the
carbon is not oxidized sufficiently, which reduces the effectiveness of
the material.
If the specific surface area is larger than 100 m.sup.2 /ton, oxidation of
carbon proceeds at an excessive rate. Consequently, the heating rate of
the base metal lags behind the rate of decarborization, since the
oxidation of carbon by solid iron oxides is accompanied by heat
expenditures and cooling of the bath. As heating professes and the bath
temperature rises, the oxidation of carbon is likely to become cyclic in
character and lead to ejections and entrainment of metal particles during
the turbulent gas release by the reaction products from the oxidation of
carbon.
Thus, according to the invention, the optimum and, therefore preferred
ratio of external surface area of oxide-containing material to the weight
per unit mass of the iron-carbon alloy is in the range of 5-100 m.sup.2
/ton.
As stated above, according to the invention, the ratio of the carbon to
silicon in the iron-carbon alloy is maintained in the range of 4-40:1. The
noted ratio is preferred since oxidation of both carbon and silicon is
attained in this range.
If the ratio of carbon to silicon is less than 4:1, silicon oxidizes
predominantly and forms a reaction product in the form of high-melting
silicon dioxide. This high-melting phase blocks the interface between the
oxide-containing material and the molten iron-carbon alloy, hindering the
supply of both oxygen and carbon to the reaction fronts. Consequently, the
oxidation of carbon is abated.
If the ratio of carbon to silicon is greater than 40:1, carbon
predominantly oxidizes until the C/Si ratio reaches equilibrium values
corresponding to this temperature. At this point, there is no heat release
in the reaction zone. In view of the endothermic nature of the oxidation
of carbon by solid oxidizing agents, the oxidation of carbon slows or even
abated. Consequently, the melting time is lengthened.
Referring to Table 1, there is shown the results of experimental melts of
the charging stock of the invention in an electric furnace.
TABLE 1
__________________________________________________________________________
Heats in experimental
Composition electric fumaces
of pig charge Rate of oxidation of
Ratio of external surface are
carbon at reduced
of oxide-containing material
Ratio of carbon to
temperatures in initial
to weight of iron-carbon
silicon in iron-carbon
melting period,
Item No.
alloy alloy %/min
__________________________________________________________________________
Prior art
3.0 5.0 0.2
1 5 4.5 0.27
2 50.0 6.0 0.4-0.5
3 75.0 12.2 0.5-0.6
4 100.0 25.6 0.6-0.8
__________________________________________________________________________
The change in carbon content (›c!, %) in the charging stock and the rate of
carbon oxidation (V.sub.c =.differential.›c!/.differential..tau., %/min)
was recorded for different ratios oft he external surface area of the
oxide-containing material to the weight of the iron-carbon alloy (see
FIGS. 1, 2, and 3).
Referring to FIG. 1, there is shown the change in carbon content (›c!, %)
in the charging stock (from C initial, ›C!.sub.i, to C final, ›C!.sub.f)
as a function of the carbon oxidation rate (V.sub.c
=.differential.›c!/.differential..tau., %/min) when the ratio of the
external surface area of the oxide-containing material to the weight of
the iron-carbon alloy is 5 m.sup.2 /ton.
FIGS. 2 and 3 graphically illustrate the results of the ratio of external
surface area of oxide-containing material to the weight of the iron-carbon
alloy of 35 m.sup.2 /ton and 100 m.sup.2 /ton, respectively.
As illustrated in FIGS. 1-3, as the heating rate of the given charging
stock increases (with increasing ratio of the external surface area of the
oxide-containing material to the weight of the iron-carbon alloy), the
nature of the carbon oxidation changes. At low ratios of external surface
area of the oxide-containing material to weight of the iron-carbon alloy
(See FIG. 1), the rate of carbon oxidation has two pronounced peaks: one
at 1240.degree. C., and the other at 1400.degree. C. The first peak
corresponds to the point of initial melting of the pig iron and carbon
oxidation in the liquid iron by the oxygen in the solid pellets. At this
point the charge material is in a solid-liquid state while preserving the
original structure. The solid pellets are then covered with a film of
silicon dioxide (due to the oxidation of the silicon in the pig iron) and
the carbon oxidation reaction slows. As the temperature rises to
.about.1400.degree. C., the base of the pellets (hematite gains) melts and
the reaction reaccelerates. However, conventional carbon oxidation in the
liquid iron (via the oxygen contained in the ferruginous slag) is already
proceeding.
If the ratio of the external surface area of the oxide-containing material
to the weight of the iron-carbon alloy is 35 m.sup.2 /ton, the heating
rate increases, the peaks reflecting the carbon oxidation rate converge
and, at high heating rates (See FIG. 3), merge. The absolute value of the
carbon oxidation rate is approximately 0.2-0.8%/min under these
conditions, which is approximately double the maximum rates of carbon
oxidation in a basic oxygen furnace.
Under laboratory conditions, when the ratio of the external surface of the
oxide-containing material to the weight of the iron-carbon alloy is
approximately 100 m.sup.2 /ton, the melt boils up energetically and the
volume of the slag-metal-gas emulsion increases. Thus, a timber
distinctive characteristic of the charging stock of the invention is the
presence of a first period of carbon oxidation: oxidation of the carbon in
the pig iron by the oxygen in the solid pellets. The three-dimensional
structure of the charging stock is basically preserved in this case,
ensuring a high specific surface of reaction.
In further experiments conducted by Applicants in a basic oxygen furnace, a
charging stock based on basic pig iron (78-83 wt. %) and iron-ore pellets
(17-22%) with a (5-20):1 ratio of the external surface of the iron-ore
pellets to the weight of the iron was employed. The content of ore pellets
was chosen so that the amount of oxygen in the pellets would be less than
the stoichiometric value corresponding to the reaction of complete
oxidation of the carbon in the charging stock.
The chemical composition (calculated) of one exemplar composition of the
charging stock (20% pellets by weight) is presented below (in percent):
______________________________________
Fe met.
FeO Fe.sub.2 O.sub.3
SiO.sub.2
Al.sub.2 O.sub.3
CaO MgO MnO Na.sub.2 O
______________________________________
74.540
0.630 17.534 1.712
0.004 0.730
0.089
0.015 0.003
______________________________________
K.sub.2 O
C Si Mn Cr Cu Ni P S
______________________________________
0.013 3.712 0.635 0.277
0.015 0.007
0.009
0.057 0.018
______________________________________
As will be recognized by one having ordinary skill in the art, the amount
of oxygen in the charging stock that is employed for oxidation of the
carbon and silicon is approximately 87.5% of the mass required for the
reactions (See Table 2).
TABLE 2
__________________________________________________________________________
Oxygen Inflows and Distributions for Oxidation of Carbon and Silicon
of the Charging Stock (Weight of Charging Stock Is 1 ton)
Amount of oxygen from bath and
blowing
Oxygen in iron oxides, kg
Oxygen consumption for reactions, kg
that goes into reactions,
__________________________________________________________________________
kg
FeO (0.630/100) .times. (16/72) .times. 1000 = 1.4 kg
C--CO 0.9* .times. (3712/100) .times. (16/12) .times.
1000 = 61.699-54.002 = 7.697 kg
44.544 kg
FeO (17.534/100) .times. (48/160) .times. 1000 =
C--CO.sub.2 0.1 .times. (3.712/100) .times. (32/12)
.times. 1000 =
52.602 kg 9.898 kg
Si--SiO.sub.2 (0.635/100) .times. (32/28) .times. 1000
=
7.257 kg
TOTAL: 54.002 kg TOTAL: 61.699 kg
__________________________________________________________________________
*0.9 is the fraction of the carbon that goes to CO.
TABLE 3
__________________________________________________________________________
Content of Residual Elements in Isotropic Electrical Steel
as a Function of Charge Composition, and the Yield of Higher Steel
Brands
Yield of higher steel brands, %
2312, GOST 21427.2-
2212, 2215, 2216,
83, 23.15 Technical
2412, 2413, 2414,
Charge composition, %
Content of elements, %
GOST ›USSR State
Specifications TU 14-
2415, GOST 21427.2-
production period)
Cr Ni Cu Standard! 21427.2-83
106-335-89
83
__________________________________________________________________________
Liquid iron 79.1
0.037
0.046
0.064
76.2 72.5 55.0
Scrap 20.9
Liquid iron 83.1
0.029
0.31
0.038
84.7 74.2 68.7
Scrap 16.9
Liquid iron 90.3
0.022
0.027
0.026
90.6 91.6 85.0
Scrap 6.0
Superkom 3.7
__________________________________________________________________________
With the noted distribution of oxygen (for carbon oxidation reactions),
when some of the oxygen (.about.10-15% of the oxygen involved in the
reactions) evolves from the liquid bath during blowing, the process of
oxygen buildup in the converter bath is virtually eliminated. By virtue
oft he melting point of the charging stock
(.about.1200.degree.-1250.degree. C.), which is lower than that of the
scrap, the process of decarborization of the bath begins earlier and
initially proceeds inside the charging stock being melted, with evolution
of carbon monoxide.
Further, as illustrated in FIG. 4, the use of the charging stock of the
invention (denoted 2) accelerates the oxidation of carbon in the first
cold period of the heat by approximately 25-100%. At the end of this
period (.about.4 min) the rates of carbon oxidation of the charging stock
2 and the conventional charge material 1 are approximately equal, and from
this time forward the oxidation of carbon proceeds more uniformly than
with the conventional charge material. In this instance, by virtue of the
use of the improved charging stock, peak gas releases are eliminated,
thermal loads on the equipment of the boiler-cooler are lowered, and
entrainment and metal-coating of boiler elements are reduced.
The cooling effect of the charging stock of the invention is also close to
that of scrap. Therefore, as will be recognized by one having ordinary
skill in the art, when the charging stock is introduced into the metal
heat the consumption of coolant additives can be determined with relative
ease. Replacing conventional scrap with the charging stock of the
invention in a 1:1 ratio thus leads to essentially no change in the
thermal balance of the heat.
The charging stock of this invention thus makes it possible to reduce the
content of residual elements in isotropic electrical steel and to increase
the yield of higher steel brands (see Table 3) when the stock is used as a
replacement for metal scrap in the burden of a converter.
As will be recognized by one having ordinary skill in the art, several key
advantages can be realized through the use of the improved charging stock
of the invention. The advantages include the advanced creation of a highly
developed surface on the oxide-containing material. This sharply
accelerates the oxygen supplied to the carbon reaction front. The increase
in the rate of oxygen transport is so significant that it ceases to limit
the reaction of carbon oxidation. The reaction is thus limited by the rate
of heating and melt-down of the charging stock.
An additional advantage of the charging stock is that early oxidation of
carbon is ensured at the start of the heat at reduced bath temperatures
(1200.degree.-1300.degree. C.). Moreover, extremely high values of carbon
oxidation rates at reduced temperatures--values corresponding to
conditions under which pig iron is blown with oxygen (0.2-0.4% C/min)--can
be attained.
Finally, the presence of excess oxygen at the interface between the
oxide-containing material and the iron-carbon alloy melt as a result of
the large surface area of the oxide-containing material, the high oxygen
concentration in the oxide-containing material and its activity facilitate
the simultaneous, parallel oxidation of all elements that, under these
conditions, have a greater affinity for oxygen than iron, including carbon
and silicon. This substantially reduces or eliminates the dependence of
the percent carbon oxidation on the silicon content in the iron-carbon
alloy, especially at reduced temperatures.
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